insulation question

Hi everyone,

Homeowner who has been reading a ton on this board and learning a lot.
I have a 3rd floor walk up attic that I am considering finishing off part of it. The finished part would be about 20x20. The roof pitch is 9/12.
Here is a pic of the space:

Here is the current insulation (house is 1993):

My HVAC is in the attic and the duct work is outside the knee wall. (hence I can finish the 20x20 space). From what I have been reading, having my HVAC in conditioned space would be better. So I am thinking of insulation the rafters, with baffles from soffit to ridge. Granted I am looking at 27 “rafter bays” front and rear (54) and each is 24 feet in length. That is 1300 feet of batts not including the side walls!
Anyway my main question is where the knee wall intersects with the rafters (2x8s). There is only 5" from the top of the knee wall to the roof decking at the intersection. Even if I put furring strips on the rafters for more depth for insulation, what do I do there???
Here is a pic:

Any thoughts? Am I crazy to insulate the whole attic space? Should I just focus on the “room”?

Where do you Live.

If it were my home I would use a spray foam insulation in the rafter spaces instead of using batts. You get a better R=Value and it acts as a vapour barrier. Just my two cents

You need an engineer .

I live in Richmond, VA

That is an option, agreed, but I have heard that if you have a roof leak of any kind, once you spray foam, you cannot “pull it away” and look…

Can you elaborate?


  1. Make sure that you check the attic floor joists and make sure that they are big enough to support a live load. Many times, ceiling trusses are undersized because the “attic” is seen only as storage / utility space.
  2. Make sure that any furnace in the attic is a cat 4, high efficency furnace that takes its combustion air from the outside, NOT from in the attic.
  3. Spray foam, CLOSED CELL spray foam, applied on the underside of the roof decking is the onoly way to go. 2 - 3 " is all you need. Seal up the attic space.
  4. As previously stated, DO NOT try to do the work yourself, unless you are a licensed GC and have an Architect or Engineer look things over.

BTW: What you have on the floor of the attic is NOT insulation (regardless of what others may call it). It is merely an air filtration system. It does not stop air movement as real insulation should.

Hope this helps;

Many shingle manufacturers will not honor their warranties with this.

That is their right, although I have no idea why. Good insulation under the shingles will certainly extend their service life. Plus, there is the decking between the shingles and the insulation.

Look, the concern was that closed cell would not allow a leaking roof to show itself, through leakage into the attic, and rot the roof structure. What has been found is that the water runs to the eave and leaks there.

Not true because with foam there is no need for soffit ventilation which keep the shingles cool.

Soffit (and roof) ventilation is more for management of humidity, not heat. If the attic deck is insulated, there is no heat coming up from the attic area under the shingles. The temp of the decking will be cool with any heat coming the shingles, not from the attic.

Basic thermodynamics and building science. Trust me, I used to be a physicist.

If the shingles cannot take the heat of the sun, that is a defect in the shingle. If the attic is properly ventilated (rare, in this area) any sunlight heat would already be traveling through the shingle and causing deterioration anyway. That is what the shingle granules are for. Now, if people install a dark colored shingle, that can be a problem because dark shingles absorb more heat and shorten the life of the shingles. But the same would happen if the attic is ventilated or not.

The damage to the shingles occurs when they are heated. If the attic is ventilated, the heat still has to travel through the shingles. Closed cell insulation or not, the same amount of heat would transverse.

Hope this helps;


That is why proper attic ventilation is key in shingle life. I’ve see damaged shingles to many time because of inadequate venting.

Good thing you got out of it.


As a Floridian, you are well aware of humidity problems in attics. Also, different areas of the country have their own specific conditions. I would posit that attic ventilation is more for humidity problems than for keeping the shingles cool.

And, I don’t believe that “whatever” is a valid argument.

In any case, we do closed cell foam under the roof deck all the time, up here, and I have not seen any decreased shingle life. Just the opposite, in fact.

Hope this helps;

Well… :slight_smile:

I do enjoy a healthy discussion and I thank everyone that has replied, yet I believe I am still at a crossroads! As a side note I am near the end of life of my roof - original 1993. Does that give me any other options if I tear off the shingles? (Professional of course).

Oh - and here is a pic of the house, to give more context:

  1. Good point - thank you and I will check - believe they are 2x10s but will verify.

2)The existing attic HVAC that serves the 2nd floor is brand new Trane unit installed in August. The return is in the 2nd floor ceiling (attic floor). Recommendation from HVAC guys was to install a minisplit system for the attic so the 2nd floor and attic room are not fighting each other since the thermostat is on the 2nd floor.

  1. Understood - if I go spray foam. :slight_smile:

  2. Heck no! Licensed professional for any spraying! Baffles and batts, I can do!


Whats the required R value for sloped ceilings where you live? It’s R28 here in Canada, so if its anywhere near there, you will have to build your rafters down roughly an additional 2.5" to be able to install R 28 batts and maintain your 2.5" airspace. If thats the case, foam may actually be more cost effective. If your required R value is low enough that you are able to install the batts in the existing cavity and still maintain your air space, there are 2 things you can do to deal with your 5" clearance at the knee wall.

  1. you could make a box out of styrofoam that runs 90 degrees to the slope of the roof, a couple inches before and after the knee wall and install cardboard dams 2.5" lower than the roof sheathing and get a spray foam company to just fill that small 6" x 5" area along the length of the roof. then you can just butt your batts up to that since it will then be square, and your cost will be substantially lower then if you had foamed the entire roof. Thats how my company often does story and a half houses to balance cost with energy efficiency. We also do this at the eave where the rafters hit the plate, since its nearly impossible to do a good job of cutting a fiver glass batt at a 37 degree angle.

  2. If you don’t want to get a spray foam company in whatsoever, you could put 3" of styrofoam above the plate, than cut styrofoam sheets at a 37 degree angle on the bottom side of the knee wall plate, and a 63 degree angle on the top side of the plate to but up against the plate. if you make your start and end flush with each other, then you can again just butt your batts up against that.

…Not sure how clear I made the description. A picture really would have been worth those thousand words. Let me know if you have any questions.

Edit: And please make sure that your floor joists are actually floor joists and not ceiling joists before you get too carried away.

The required R-Value is 30 for my area - Zone 4 - Richmond, VA. Just talked to one of the county inspectors.

Do I need a 1" or 2.5" air gap? 2.5 seems large?

If I am 2x8 rafters, that gives me 7.25" to work with.
To meet R-30, can I do the following:

  1. Use 1" baffles, then use R-19 faced batts(6.25" thick). That uses all my space in the rafter - Could I then use 2" foam board ( with R-10 value in sheets across the rafters?

Has everyone forgotten the late 1970’s? After the first energy crisis many older homes with no insulation were ‘upgraded’ by injecting foam in walls. If the foam was not mixed properly on site it would off gas formaldehyde. The media told everyone that this was very bad. This resulted in having to disclose that foam was installed, resulting in a house that could not be sold. Turned out that pretty much all manufactured wood building products off gas formaldehyde, but this did not matter in the ensuing panic to dump any house with spray foam insulation. A lot of them were simply torn down.
There probably wasn’t much wrong with the foam back them, as for today, old foam will have off gassed all of its formaldehyde. The problem was fly by nighters who mixed the two part chemicals used to create foam incorrectly, but all foam got tarred with the same brush by uninformed media and tort lawyers out to make a bundle. Me I, would never use it because those who won’t learn from history will be doomed to repeat it :slight_smile:

If you insulate the attic sheathing it becomes a wall, in other words insulate sheathing and trusses or rafters the same as you would a a wall, fill the space with insulation of your choice, put a vapor barrier on the warm side, and make sure the cold side can breathe.